1. Field of the Invention
The present invention is related to an image forming apparatus and an image forming method capable of forming color and monochromatic images.
2. Description of Related Art
An image forming apparatus based on electronic photography such as a copying machine or a printer forms a color image by transferring and overlaying images developed using four colors of toners, i.e., yellow (Y), cyan (C), magenta (M) and black (K), sequentially on a paper. This forming process of color images is called the four cycle process as it requires the developing and copying processes four times each in order to overlay the four color images. On the other hand, the forming of a monochromatic image is called the one cycle process as it requires developing and transferring processes only once.
In certain cases, data used for image forming may contain both color images and monochromatic images. This corresponds to a case where the original document consists of both color pages containing color images and monochromatic pages composed of monochromatic images. Furthermore, with reference to the printer, it corresponds to a case where the output data consists of monochromatic pages consisting of monochromatic images such as text data, and color pages including color images such as color photographs or color graphics.
In case of conventional copying machines and printers, image forming for monochromatic pages and color pages are treated by the one cycle process and the four cycle process respectively. Therefore, if the job data has both monochromatic pages and color pages, the one cycle process and the four cycle process have to be switched from one to another in the middle of the printing.
However, process switching requires changes of the process speeds such as the rotating speeds of the photoconductor drum and the transfer drum, the fixing speed, the paper transport speed, or the like, so that it causes waiting time. Process switching occurs more frequently particularly when color pages and monochromatic pages are sequenced randomly. As a consequence, the waiting times are accumulated and the printing time gets longer.
Publication of Unexamined Japanese Patent Application, JP-A-8-274974 discloses a method of solving this kind of problem using a first printer for color pages and a second printer for monochromatic pages. The method includes the steps of dividing the job data into color images and monochromatic images, printing color images and monochromatic pages by the first and second printers respectively, inserting monochromatic pages outputted from the second printer into an external paper feed unit of the first printer, and discharging color pages and monochromatic pages in a proper order from the first printer.
However, the method is naturally not applicable to a case where there is only one printer as it requires two printers. Moreover, the method is not applicable to a case in which a copying machine is to print a document containing both color pages and monochromatic pages.